European Commission President Jean-Claude Juncker has called an extraordinary meeting of several European leaders on Sunday to tackle the migrant crisis in the western Balkans as thousands trying to reach Germany are trapped in deteriorating conditions.

The Commission said in a statement on Wednesday that Juncker had invited the heads of state or government of Austria, Bulgaria, Croatia, the former Yugoslav Republic of Macedonia, Germany, Greece, Hungary, Romania, Serbia and Slovenia.

The summit comes as hordes of refugees and other migrants camp by roads in western Balkan countries in worsening autumn weather after Hungary sealed its borders with Serbia and Croatia, causing a chain reaction in other overwhelmed states.

Slovenia passed legislation on Wednesday to give the army more power help police guarding the state border as refugees streamed in from Croatia on the way to Austria, which is within the passport-free Schengen zone.

"In view of the unfolding emergency in the countries along the Western Balkans migratory route, there is a need for much greater cooperation, more extensive consultation and immediate operational action," the Commission statement said.

The summit's aim was to agree "common operational conclusions which could be immediately implemented", it said.

A 28-nation EU summit agreed last week on a range of measures to strengthen external border protection and offer financial incentives to Turkey and other nations to keep refugees on their soil and deter them from heading to Europe.

That appeared to be an effort by Juncker to raise pressure on central and southeastern European countries to cooperate among themselves in managing the migration flow and end a series of unilateral actions that have raised political tensions without providing minimum humanitarian conditions for the refugees.

The meeting will take place between 1600 and 1900 CET (1500-1800 GMT) on Sunday, followed by a working dinner and will also involve United Nations High Commissioner for Refugees, Antonio Guterres, the European Asylum Support Office and EU border agency Frontex.

The president of the European Council, Donald Tusk, and Luxembourg, which holds the EU's rotating presidency, were also invited.